The present invention relates to a backing plate to be used when using a low thermal expansion material as a sputtering target material, and particularly a backing plate obtained by diffusion-bonding an anticorrosive metal and Mo or a Mo alloy and capable of preventing warpage (deformation) during the bonding of a sputtering target and a backing plate or during sputtering, as well as a sputtering target-backing plate assembly provided with such a backing plate.
In recent years, various thin films are now required in the application of semiconductor devices, and demands of low thermal expansion materials such as silicon, germanium, and carbon are also increasing. Moreover, as the diameter of silicon wafers to be used as the substrate increases to 200 mm, 300 mm, and 450 mm, sputtering targets also grow in size, and there are needs to produce large-diameter targets also based on the foregoing low thermal expansion materials.
Meanwhile, the miniaturization of semiconductor devices has advanced to the nano region, and there are demands for depositing thin films that are thinner than conventional thin films and at a uniform film thickness across the entire substrate. Thus, the control of warpage of sputtering targets is also becoming stricter.
When producing a target formed from a low thermal expansion material, as the diameter becomes larger, there is a problem in that the difference in the thermal expansion between the target material and the backing plate material increases, and the target is prone to warpage.
Furthermore, in recent years, sputtering is being performed at high power to improve the production efficiency. Issues that become problematic during this kind of high power sputtering are the strength and cooling capacity (favorable thermal conductivity) of the backing plate itself and the bonding strength of the backing plate and the target.
In particular, when brazing a low thermal expansion target material and a generally used copper backing plate by heating the target material and the backing plate to a temperature that is equal to or higher than the melting point of a brazing material and then cooling the target material and the backing plate, the target material will become warped (deformed) in the shape of a protrusion since the copper backing plate having a greater coefficient of expansion will shrink more, and this warpage (deformation) becomes a factor that obstructs the formation of a uniform film during sputtering. Moreover, the difference in shrinkage of both materials also becomes a problem in deteriorating the bonding strength at the bonded interface.
In the application of recent semiconductors in which high quality thin film are being demanded more stringently, there were needs to resolve the foregoing problems. Furthermore, in addition to having favorable sputtering characteristics, demanded are sputtering targets having a backing plate with superior anticorrosive properties that will not contaminate the clean room or cooling water.
As conventional technologies, there are some examples where Mo (molybdenum), which has a relatively low thermal expansion among metal materials, is used as the bonding layer or the backing plate. As one such example, there is Patent Document 1. Paragraph [0006] of Patent Document 1 describes that, since warpage arises from the difference in the coefficient of thermal expansion between the target member and the backing plate during the cooling process after being bonded, and the target member becomes cracked due to such warpage, the object of Patent Document 1 is to provide a sputtering target with minimal warpage and superior cooling efficiency (paragraph [0008]). Nevertheless, this solution is a method of mixing a metal powder, such as molybdenum powder, in the metal bonding material layer, and is a method that is completely different from the present invention described later.
Paragraph [0006] of Patent Document 2 describes that a support plate material (backing plate) should be selected in consideration of its strength, anticorrosive properties, and heat-transfer properties, and offers molybdenum having low conductivity as one option of the support plate material as a material for reducing the eddy current in order to generate more stable plasma (refer to paragraph [0007], paragraph [0027]).
Nevertheless, when a molybdenum backing plate is used and cooling water is caused to flow so that it directly comes into contact with the molybdenum backing plate, rust of molybdenum oxide will easily arise, and contaminate the surrounding environment of the sputtering equipment. However, Patent Document 2 offers no description whatsoever regarding the means for improving the anticorrosive properties.
Moreover, Patent Document 3 describes a Si target which uses Mo or a Mo/Cu complex as the backing plate. Furthermore, Patent Document 3 describes that the backing plate material should be selected among materials having a thermal expansion coefficient (CTE) that is approximate to that of silicon. Nevertheless, while various methods may be considered for producing the Mo/Cu composite material, Patent Document 3 offers no description regarding the specific method of solution.
Moreover, Patent Document 3 offers no description regarding anticorrosive properties or the thickness of Mo/Cu, and it is considered that the intended requirement specification of the sputtering target cannot be achieved merely by compounding Mo and Cu.